Formation mechanism, thermal and magnetic properties of...

NANOSYSTEMS: PHYSICS, CHEMISTRY, MATHEMATICS, 2018, 9 (5), P. 676–687

Formation mechanism, thermal and magnetic propertiesof (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ (m = 4 – 7) ceramics

N. A. Lomanova1, M. V. Tomkovich1, V. L. Ugolkov2, M. P. Volkov1, I. V. Pleshakov1,V. V. Panchuk3,4, V. G. Semenov3,4

1Ioffe Institute, 26 Polytekhnicheskaya Str., St. Petersburg194021, Russian Federation

2Grebenshchikov Institute of Silicates Chemistry RAS, Adm. Makarova emb. 2, St. Petersburg,199034, Russian Federation

3St. Petersburg State University, 7-9 Universitetskaya Emb., St. Petersburg, 199034, Russian Federation4Institute for Analytical Instrumentation of RAS, St. Petersburg, ul. Ivana Chernykh, 31-33, lit. A.,

198095, Russian Federation

[email protected]

DOI 10.17586/2220-8054-2018-9-5-676-687

Specific features of the formation of Aurivillius phases (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ (m = 4−7; x = 0.0−0.7) with a perovskite-

like block having a nanometric thickness (h) of 2 – 3 nm are described. It has been established that the degree of isomorphous substitution in

the bismuth sublattice and thermal stability of phases tend to reduce with the increasing h. It has been demonstrated that the magnetic ions

inside the perovskite-like block can have antiferromagnetic interaction exchange that influences magnetic properties of the Aurivillius phases.

Keywords: aurivillius phases, nanolayers, perovskite-like nanoblocks, formation mechanism, thermal properties, magnetic properties.

Received: 21 August 2018

Revised: 26 September 2018

1. Introduction

The search for and design of new perovskite-like multiferroics is a promising trend in material science, asmedia possessing both ferroelectric and magnetic order are among the most highly demanded materials in moderntechnology [1–4]. The interest in layered Aurivillius phase multiferroics is determined by the possibility of thenanolayers’ structural stabilization during the formation of those nanostructured materials [5, 6].

In general, the structures of the layered perovskite-like oxides such as the Aurivillius phasesBim+1Fem−3Ti3O3m+3 consist of alternating fluorite-like {(Bi2O2)

2+}∞ nanolayers and bismuth orthoferrite-based perovskite-like blocks of {(Bim+1Fem−3Ti3O3m+1)

2−}∞ [7]. The thickness h of the perovskite-like blocksdiffers depending on the number of nanolayers m.

Bismuth orthoferrite-based (BiFeO3) materials are known to have high values of the Neel temperature(TN ≈ 370 ◦C) and the Curie point (TC ≈ 830 ◦C), and currently have very attractive properties from amagnetoelectronics point of view. The main interest in the design of the Aurivillius phase-structured materialsis also associated with the possibility of controlling their functional properties by varying the composition andthickness of the perovskite-like blocks up to nanometric values [1–6, 8–23]. In the Aurivillius phases with m=7-9,the perovskite-like block has a thickness of 3 – 4 nm [5].

In a unit cell of Bim+1Fem−3Ti3O3m+3, titanium and iron ions have octahedral structurally nonequivalentpositions in the inner and outer layers of a perovskite-like block. It is shown in [6, 9–17] that the distributionof cations within a perovskite-like block depends on m and may affect stability and properties of the Aurivilliusphases. In particular, it can influence the conductivity of these compounds and their magnetic behavior [13,14,17].

The Aurivillius phases’ thermal behavior, phase equilibria and transformations in binary and quasi-binarysections of the Bi2O3–SrO–TiO2–Fe2O3 system were studied in [10, 17–21]. It was demonstrated for the non-doped Aurivillius phases Bim+1Fem−3Ti3O3m+3 that an increase in the perovskite-like block thickness up tothe nanosize values leads to a decrease in their thermal stability. The Aurivillius phases Bim+1Fem−3Ti3O3m+3

have the values of the temperature of transition to the magnetically ordered state (Neel point, TN ) within the77–360 K range, and the ferroelectric transition temperature (Curie point, TC) within the range of 923 – 1023 K[11,14,15,17,21–26]. Along with that, magnetic ordering may be expected around room temperature in substanceswith h ≥3 (m ≥7) [14]. Above TC (923–1023 K), the structures of the Aurivillius phases Bim+1Fem−3Ti3O3m+3

are tetragonal, while in the ferroelectric phase the crystal lattice is either orthorhombic or monoclinic [15, 24].

Formation mechanism, thermal and magnetic properties... 677

The influence of isomorphous substitution in the Aurivillius phase structures on their phase transition temperaturesrequires systematic study.

The methods of synthesizing simple bismuth-containing perovskites and layered perovskite-like oxides, whichmake it possible to perform the synthesis of materials while varying their morphology and particle sizes withina broad range by changing technological parameters, are described, for example, in [27–40]. It is shown thatirrespective of the technology, material formation begins at the melting temperature of the surface (non-autonomous)bismuth oxide-based phase, at which mass transfer in the reaction system is activated [41–43]. A study of theformation of phases with variable composition during the isomorphous substitution of ions in the Aurivillius phasesstructure requires refining of the mechanism of phase formation.

The present work offers a study of the formation processes, as well as of thermal and magnetic properties ofceramic materials based on the Aurivillius phases (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ with different degrees ofisomorphous substitution for strontium in the bismuth sublattice.

2. Experimental

The method of solid-phase chemical reactions was used for producing samples with a nominal compositioncorresponding to (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ (m = 4, 5, 6, 7 and x = 0.0 – 0.7).

The reagents used were the bismuth oxide, iron(III) oxide, titanium oxide and strontium carbonate (all of the99.9% purity).

The samples were heat treated in the “heating – isothermal exposure – cooling” mode. The isothermal exposurewas performed from 500 – 1000 ◦C for 5 – 80 hours at each temperature, depending on the degree of synthesiscompletion. The phase composition of the samples at each stage of synthesis was determined by X-ray diffraction(XRD-7000 Shimadzu diffractometer, CuKα-radiation). The unit cell parameters were calculated using the PDWin4.0 software package.

The microstructure and elemental composition of the samples were determined by scanning electron microscopyand elemental energy-dispersive microanalysis (FEI Quanta 200 SEM with EDAX attachment). The chemicalformula of the synthesized substance was calculated according to stoichiometry and based on the assumption thatthe oxidation states of Bi, Sr, Ti and Fe are III, II, IV and III, respectively. In order to determine the main phase,the sample area and individual smaller sites were subjected to the elemental analysis.

The samples’ thermal behavior was studied by differential scanning calorimetry (DSC) using the NETZSCHSTA 429 calorimeter, in air, with the heating rate of 10 ◦C/min. The changes in the samples’ linear dimensionswere determined by dilatometry using the NETZSCH DIL 402 E dilatometer, in air, with the heating rate of10 ◦C/min. A pellet-shaped sample, 5 mm in diameter and 3 mm thick, was used. The linear thermal dilationcoefficient (αt) was determined within the 200 – 400 ◦C range, in which phase transformations are absent.

The Mossbauer spectroscopy of the samples was accomplished using the WISSEL spectrometer in the ab-sorption geometry at room temperature, using a 57Co(Rh) source. The isomer shift values were determined withreference to the α-Fe isomer shift.

The samples’ magnetic behavior was determined using a Physical Properties Measuring System, QuantumDesign vibration magnetometer. Specific magnetization M as a function of temperature was studied in the presenceof a constant field H = 500 Oe and in the zero field cooling (ZFC) mode. The M(H) dependence was measuredat 5 and 300 K within the 0 – 50 kOe range.

3. Results and discussion

The Aurivillius phase-structured solid solutions (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ have been synthesized inthe Bi2O3–SrO–TiO2–Fe2O3 system. The value of δ can vary depending on the charge compensation characterduring formation of the (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ solid solution. If the charge is compensated at theexpense of the change in the iron ions charge from Fe3+ to Fe4+, then δ = 0, while in the case of oxygenvacancies creation δ = 0.5x(m+ 1). The possible character of the charge compensation may be determined fromthe Mossbauer data given below.

As is shown in [19], the structure of the homologous series (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ contains aperovskite-like block with the number of nanolayers m = 4 – 7 and thickness h of about 2 – 3 nm. Along with that,the limit of isomorphous substitution of bismuth for strontium (x) depends on the perovskite-like block thicknessh. The x(m) and, correspondingly, x(h) dependences are illustrated in Fig. 1. It can be noted that the substitutionof bismuth for strontium in the Aurivillius phases with the given composition occurs at an m up to ∼7 (h ∼3 nm).In this case, the isomorphous capacity of the system decreases with increasing thickness of the perovskite-likeblock to a nanosize value, and this influences the formation mechanisms of these substances, as well as thermal

678 N. A. Lomanova, M. V. Tomkovich, V. L. Ugolkov, et al.

and magnetic properties of ceramic materials based on them. The experimental results presented below have beenanalyzed in comparison with the corresponding data on the features of the solid-phase synthesis, structure andproperties of the non-doped Aurivillius phases Bim+1Fem−3Ti3O3(m+1), described in [6, 10, 14, 17, 21]. Synthesisconditions, elemental and phase compositions of the obtained samples are presented in Table 1.

TABLE 1. Synthetic conditions and sample composition

Nominal Synthesis Composition of the Aurivillius phases PhaseNo. composition m conditions (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ m∗) composition

after EDS-analysis by XRDanalysis

1 Bi5FeTi3O15 4 900 ◦C, 5 h Bi5.1±0.1Fe1.2±0.1Ti3O15.5±0.3 4.1 A

1-1 Bi4.9Sr0.1FeTi3O14.75 4 900 ◦C, 10 h Bi5.3±0.3Sr0.2±0.1Fe1.2±0.1Ti3O16.0±0.3 4.3 A




1-5 Bi4.1Sr0.9FeTi3O12.75 4 1000 ◦C, 10 h Bi3.9±0.2Sr1.2±0.4Fe1.2±0.2Ti3O14.9±0.1 4.1 A, P, M

2 Bi6Fe2Ti3O18 5 890 ◦C, 5 h Bi5.8±0.2Fe1.8±0.2Ti3O17.4±0.2 4.8 A

2-1 Bi5.7Sr0.3Fe2Ti3O17.1 5 900 ◦C, 10 h Bi5.9±0.2Sr0.3±0.1Fe2.3±0.1Ti3O18.6±0.2 5.2 A, M

3 Bi7Fe3Ti3O21 6 870 ◦C, 5 h Bi7.4±0.3Fe3.0±0.2Ti3O21.6±0.2 6.3 A

3-1 Bi6.7Sr0.3Fe3Ti3O19.95 6 900 ◦C, 20 h Bi6.8±0.2Sr0.4±0.1Fe3.2±0.2Ti3O21.4±0.3 6.2 A, M, S

4 Bi8Fe4Ti3O24 7 870 ◦C, 5 h Bi8.0±0.1Fe4.2±0.2Ti3O24.3±0.3 7.1 A, M

4-1 Bi7.7Sr0.3Fe4Ti3O22.8 7

900 ◦C, 20 h Bi7.5±0.1Sr0.4±0.1Fe4.0±0.2Ti3O23.7±0.1 7.0 A, M

870 ◦C, 10 hBi6.5±0.1Sr0.4±0.2Fe2.2±0.2Ti3O19.5±0.3 5.5

A, PBi7.5±0.1Sr0.4±0.1Fe3.2±0.1Ti3O22.5±0.5 6.5Bi1.1±0.1Sr0.1±0.1Fe0.7±0.1Ti0.3O3.4±0.2 —

∗) average value from EDS- analysis. Notation: A – Aurivillius phases, P – BiFeO3, M – Bi2Fe4O9, S – Bi25FeO39

FIG. 1. Scheme for isomorphous substitution of bismuth by strontium in Aurivillius phases(Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ structure

X-ray diffractograms of the Sr-doped Aurivillius phases (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ , presented inFig. 2 illustrate the phase composition of the samples at a synthesis stage from 500 – 1000 ◦C. At 500 – 600 ◦C,that is, after melting of the bismuth oxide-based surface phase [28,33] and hence, after activation of mass transferin the reaction system [41–43], α-Bi2O3 interacts with titanium and iron oxides yielding the main phase Bi25FeO39.


At 700 ◦C, small amounts of BiFeO3 and Aurivillius phases start to appear in the reaction system (Fig. 2). Thesillenite-structured Bi25FeO39 and perovskite-structured BiFeO3 are the intermediate products in the synthesis ofthe Aurivillius phases. Their formation in the reaction system facilitates the formation of a layered perovskite-like structure with a predetermined composition due to a gradual rearrangement in the first coordination sphereof bismuth [28]. According to the quantitative X-ray phase analysis, the share of BiFeO3 increases along withthe increasing Sr content in the reaction system, and with the increasing h. At temperatures above the initiationof Bi25FeO39 decomposition (above 700 ◦C), diffractograms of the samples with m = 4 show reflexes of onlyAurivillius phases and BiFeO3, their ratio being the evidence of the gradual formation of a layered perovskite-likestructure. Formation of the target product occurs at temperatures above that of metastable BiFeO3 decomposition(855±5 ◦C [39]).

FIG. 2. X-ray diffractograms (λ= 1.54056 A) of (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ after ther-mal treatment at 500, 600, 700, 800, 850, 870 and 900 ◦C: a – Bi4.7Sr0.3FeTi3O15−δ (m = 4);b – Bi6.7Sr0.3Fe3Ti3O21−δ (m = 7)

Diffractograms of the multilayered Aurivillius phases with m > 5 (h > 2.5 nm) show that the process ofBi25FeO39 decomposition proceeds gradually up to 850◦C, and the substance of this phase is spent to increase theshare of BiFeO3 in the reaction system. At 870 ◦C, the diffractogram of a Bi7.7Sr0.3Fe4Ti3O24−δ sample withm = 7 shows that the two main products are (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ and the perovskite-like BiFeO3.It should be noted that the reflexes observed at the intermediate stages of synthesis correspond to the Aurivilliusphases with a smaller m, which is confirmed by the elemental analysis for the sample with m = 7, heat treated at870 ◦C (Table 1).


When the temperature is further increased, the perovskite-like BiFeO3 builds into the layered structure, and itleads to the formation of the target product (Bi7.3Sr0.3Fe4Ti3O24−δ) with an output around 95%. Some quantity ofthe impurity phases on the basis of Bi25FeO39, and Bi2Fe4O9 resulting from its decomposition, are observed at thefinal stages of (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ synthesis. Their quantity is increasing along with the numberof layers m and, hence thickness of the nano-sized perovskite-like block. In contrast to the Sr-doped Aurivilliusphases, the Bim+1Fem−3Ti3O3(m+1) homologous series can have a higher degree of homogeneity, having beensynthesized at a lower temperature and a shorter isothermal exposure (see Table 1).

X-ray diffractograms of the obtained materials with the phase and elemental composition corresponding to thetarget product output of over 90 % (see Table 1) are presented in Fig. 3(a,b). It may be noted that the Sr-dopedAurivillius phases with m > 5 perovskite-like layers contain a small number of impurity phases (Fig. 3b). Thedata on the orthorhombic unit cell volume (Table 2) suggest that it poorly depends on the isomorphous substitutionin the structure and increases with the increasing m.

TABLE 2. Sample characteristics

No Samples mV ∗,

TK , ◦C Tdec,◦C TS , ◦C αt, 1/K

M ,emu/g,A3 at 20kOe

1 Bi5FeTi3O15 4 1226 740 [7, 23] 1100 840 [7, 20, 21] 10 [21] 0.10

1-1 Bi4.9Sr0.1FeTi3O15−δ 4 1224 740 1050 770 9 —

1-2 Bi4.7Sr0.3FeTi3O15−δ 4 1222 710 [7] 1080 765 10 0.10

1-3 Bi4.5Sr0.5FeTi3O15−δ 4 1218 700 1130 700 8 0.10

1-4 Bi4.3Sr0.7FeTi3O15−δ 4 1213 — 1150 650 9 0.11

2 Bi6Fe2Ti3O18 5 1520 690 [7, 23] 980 [7, 20, 21] 720 [7, 20, 21] 13 [21] —

2-1 Bi5.3Sr0.3Fe2Ti3O18−δ 5 1516 625 [19] 900 [19] 680 10 0.13

3 Bi7Fe3Ti3O21 6 1710 682 [7] 980 [7, 20, 21] 635 [7, 20, 21] 11 [21] 0.13 [14]

3-1 Bi6.7Sr0.3Fe3Ti3O21−δ 6 1708 620 [19] 940 [19] 660 8 —

4 Bi8Fe4Ti3O24 7 1978 665 [7] 970[7, 21] 765 [7, 20, 21] 12 [21] 0.15 [14]

4-1 Bi7.3Sr0.3Fe4Ti3O24−δ 7 1965 590 [19] 935[19] 590 10 0.14

Typical SEM images of the samples after the final stage of heat treatment (Fig. 4) show the lamellar mor-phology of the resulting ceramic materials. The elemental chemical analysis data show that the main phase in thesamples at the final stage of synthesis corresponds to the target product sufficiently well (Table 1). In order to refinethe mechanism of formation of the layered perovskite-like structure, the composition of the Bi7.3Sr0.3Fe4Ti3O24−δsample (m = 7) was analyzed in more detail at the intermediate stage of synthesis. According to the obtainedresult, at 870◦C the sample contains the Aurivillius phases (Bi0.6Sr0.4)m+1Fem−3Ti3O3(m+1)−δ with m < 7, thatis, with a smaller m value than that specified by stoichiometry (Table 1). Along with these phases, the bismuthorthoferrite-based solid solution Bi1.1Sr0.1Fe0.7Ti0.3O2.7 is recorded. Under further thermal treatment, a part of itssubstance is used in the Aurivillius phase formation, while another part is engaged in formation of a mullite-likeimpurity phase Bi2−xSrxFe4O9−x. The small quantity of this phase prevents precise determination of its compo-sition. However, it was shown in [19] that increasing the quantity of the impurity in the system, along with theincrease in m and the limited solubility of (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ , allowed a suggestion that one ofthe factors that complicates synthesis of the multilayered Sr-doped Aurivillius phases is the structural stabilizationof impurity phases under isomorphous substitution in the bismuth sublattice.

Figure 5 and Table 2 present the results of the Sr-doped Aurivillius phase’s thermal analysis. The influence ofthe isomorphous substitution in the bismuth sublattice on the thermal behavior of(Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ has been considered for the first time in [19] for the fixed x value of 0.3. Itwas shown that such a degree of substitution makes the Curie point lower if compared with that for the non-dopedAurivillius phases, and the temperature of their decomposition insignificantly higher (see Table 2). The influenceof isomorphous substitution on phase transformations and on the decomposition temperature has been studied forthe four-layered Sr-doped Aurivillius phase Bi5−xSrxFeTi3O15−δ (x = 0.0, 0.3, 0.5, 0.7), because this materialshows the maximum degree of isomorphous substitution in the bismuth sublattice (see Fig. 5a).

The DSC curves for the Bi5−xSrxFeTi3O15−δ samples show small endothermal effects in the medium tem-peratures domain (TC =650 – 750 ◦C, which correspond to a phase transition of the second kind from orthorhom-bic symmetry to tetragonal symmetry (Curie point) [22, 24]. Thermal effects in the high temperature domain


FIG. 3. X-ray diffractograms (λ= 1.54056 A) of (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ after ther-mal treatment at 900◦C: a – un-doped samples; b – Sr-doped samples. The curve numbercorresponds to the sample number (Table 1). Notation: A – Aurivillius phases, P – BiFeO3, M –Bi2Fe4O9, S – Bi25FeO39

(Tdec > 900 ◦C) are related to the Aurivillius phase decomposition [10, 18, 20, 21]. As can be seen, a peak at740±5◦C is well-expressed for the non-doped sample Bi5−xSrxFeTi3O15−δ (x = 0.0). For the Sr-doped Auriv-illius phase, thermal effects at the Curie point are more diffuse. Along with that, no significant difference isobserved for the TC value in the case of bismuth ion substitution with a larger-radius ion (rBi3+(V III)=1.25 A,rSr2+(V III)=1.39 A [44]). It follows from the analysis of high-temperature effects in DSC curves at different xvalues, that Tdec gradually grows together with the increasing degree of bismuth substitution with Sr, and in factdoes not depend on thickness of the perovskite-like block at m > 5.

The curves of the Sr-doped Aurivillius phases’ thermal expansion dL/L0(T ) are presented in Fig. 5b. Thesintering initiation temperature (TS), was determined from the maximum of the dL/L0(T ) curves. It lies withinthe 590 – 765 ◦C interval and tends to decrease with fewer fusible components in the system, that is, of bismuthand strontium oxides which localize on the grains’ surface, as a rule. As is seen from Table 2, there is a correlationbetween TS and TC , which apparently suggests the activation of diffusion in the reaction system during a structuraltransition. The coefficient of linear thermal expansion of the obtained materials is αt = 9 ± 1 · 10−6 K−1 (see


FIG. 4. SEM-images of (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ (m = 7) after its heat treatment at900 ◦C: a – un-doped samples; b – Sr-doped samples

FIG. 5. Thermal analysis: a – DSC and TG curves of (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ withm = 4; b – dilatometric curves of the samples with different m. The curve number correspondsto the sample number (Table 1)


Table 2). A comparison of the obtained αt with its values for the non-doped Aurivillius phases given in [21] showsthat it poorly depends on the degree of isomorphous substitution and the number of layers in the perovskite-likeblock.

A Mossbauer study has been performed to analyze the effect of isomorphous substitution in the bismuthsublattice on the state of iron ions and on the character of cationic distribution over the structurally nonequivalentoctahedral positions in the inner (I) and outer (II) layers of the perovskite-like block. The Mossbauer data for thestudied Sr-doped Aurivillius phases with m = 4 – 7 are presented in Fig. 6. The spectral parameters for the samplesgiven in Table 3 show that the obtained materials do not contain magnetically ordered phases at room temperature.The isomer shift (IS) and quadrupole splitting (QS) values determined for (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ(x = 0 – 0.3; m = 4 – 7), are characteristic for the Fe3+ ions in an octahedral environment (see Table 3), that is,the charge compensation at the substitution of Bi3+ for Sr2+ occurs mainly at the expense of vacancies appearingin the oxygen sublattice. The materials based on the four-layer Aurivillius phase Bi5−xSrxFeTi3O15−δ (x = 0.5,0.7) contain a small amount of iron as Fe4+, it being an evidence of the possible charge compensation in presenceof high strontium content in the solid solution, and at the expense of the conversion of some Fe3+ ions into Fe4+.

TABLE 3. Mossbauer spectral parameters for room temperature samples

Samples m DoubletIsomer shift, Quadrupole Integral intensities

mm/s splitting, mm/s ratio, %

1 Bi5FeTi3O15 41 0.37 0.60 762 0.17 0.58 24

1-2 Bi4.7Sr0.3FeTi3O15−δ 41 0.38 0.60 932 0.20 0.73 12

1-3 Bi4.5Sr0.5FeTi3O15−δ 41 0.36 0.60 912 −0.02 1.68 9

1-4 Bi4.3Sr0.7FeTi3O15−δ 41 0.35 0.59 912 0.01 1.72 9

4 Bi8Fe4Ti3O24 71 0.38 0.57 732 0.31 0.61 27

4-1 Bi7.3Sr0.3Fe4Ti3O24−δ 71 0.38 0.60 932 0.20 0.73 7

The general shape of the IS(m) and QS(m) curves for the non-doped compounds Bim+1Fem−3Ti3O3(m+1),plotted with an account for the parameters of spectra of iron ions in positions I and II, is described in [6,9,11]. TheSr-doped Aurivillius phases show similar dependences, however with a number of peculiarities. For instance, forthe inner layer of the perovskite-like block (I) with the highest concentration of the magnetic ions, the IS value isindependent of both m and the degree of isomorphous substitution x. The outer layers of the perovskite-like block(II) in the Aurivillius phases are presumably subjected to structural distortion, and with m >5 significantly differin the IS and QS values (Fig. 6a,b). This is especially expressed in the Aurivillius phase with m = 4, the structureof which contains only 25 % Fe3+ ions, which are apparently localized predominantly at the internal positions ofthe perovskite-like block (Fig. 6). The isomorphous substitution in the bismuth sublattice insignificantly deflectsthis distribution from the ideal, while for the Aurivillius phases with m > 5 this deflection is more apparent.Therefore, the difference between the average charge state and symmetry of the environment of iron ions inpositions I and II is more noticeable in solid solutions with m = 4 if compared with the phases with m ≥ 5. Themultilayered Aurivillius phases have iron ion content > 40 %, which are apparently distributed less orderly withinthe perovskite-like block.

The IS values determined for the iron ions in positions I and II converge for the non-doped Aurivillius phaseswith m > 5. The IS value for positions I and II poorly depends on m for the phases (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δwith x = 0.3. Apparently, such a degree of bismuth substitution for strontium has no influence on the effectivecharge of iron ions in the perovskite-like block inner layer and weakly increases the charge in the external layers.As was previously mentioned, the Bi5−xSrxFeTi3O15−δ solid solutions (m = 4) have a large isomorphous capacity,and the dependence of IS and QS on x is more pronounced in them (see inserts in Fig. 6a,b). It can be seen thatthe IS and QS values determined for the iron ions in the inner position I, are also independent of x. The greatestdifference in the value of the iron ions effective charge is observed for the external position (II) at x=0.5, 0.7


FIG. 6. Mossbauer spectral parameters of (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ: a – the depen-dence of isomer shift (IS) value on m (insert: the dependence of IS of the Aurivillius phasewith m = 4 on x); b – the dependence of quadrupole splitting (QS) value on m (insert: thedependence of QS of the Aurivillius phase with m = 4 on x). 1 – un-doped Aurivillius phases;2 – Sr-doped Aurivillius phases with m = 4 – 7, x = 0.3; 3 – Sr-doped Aurivillius phases withm = 4, x = 0.5; 4 – Sr-doped Aurivillius phases with m = 4, x = 0.7

(see insert in Fig. 6a), which is accompanied by an increase in the symmetry of their environment (see insert inFig. 6b). Apparently, this may indicate the dominating localization of strontium atoms in the outer layers of theperovskite-like block.

The results of magnetic measurements of the obtained materials are shown in Figs. 7,8 (for the samples’composition, see Table 1). It can be seen that at 5 K, the samples have a pronounced dependence of theM(H) magnetization curves shape on m. For the samples with m > 5, regardless of the degree of isomorphoussubstitution, there is a deviation from linearity that decreases with the increasing h. It should be noted that even atlow temperatures and very large fields, magnetic saturation is not achieved. For the Aurivillius phases with m > 5,the M(H) dependences are close to linear. At T = 5 K, the hysteresis loops are absent in all samples, whichcan indicate their paramagnetic state at any temperature, however a noticeable deviation from linearity for thecompounds with m ≤ 5 suggests that this is not true, at least for these samples. When the temperature increases upto 300 K, all the M(H) dependences become linear. At 300 K, the magnitude of the Aurivillius phase’s magneticmoment obtained in the present work, as well as of those described in [14], is 0.1 – 0.2 emu/g at 20 kOe (seeTable 2). As can be seen, the essential differences in their composition and, correspondingly, the differences in theFe3+ magnetic ion content in the structure, practically do not affect the value of M near room temperature.

Figure 8a shows the temperature dependences of the specific magnetization M(T ), the shape of which istypical for paramagnets. The values of the magnetic responses differ significantly in this case. For instance, theAurivillius phases with m = 7 have approximately four-fold less magnetization at T = 5 K than the phases withm = 4. However, the analysis performed on the reverse temperature dependences of the magnetization 1/M(T )(Fig. 8b) shows that it is impossible to isolate a linear section, that is, it is impossible to accurately describe theobtained curves by the Curie–Weiss law.

Nevertheless, it is possible to note their similarity, from which only the m = 7 Aurivillius phases sharplydeviate. The violation of the strictly paramagnetic behavior can be due to the presence of a magnetically orderedstate or, more likely, magnetic fluctuations. Although a number of studies have reported that Bi5Ti3FeO15 (m = 4)demonstrates paramagnetic behavior without the long-range magnetic order up to very low temperatures [16, 26],there are indications of the appearance of an antiferromagnetic transition in the 50 – 80 K range. A theoreticalanalysis of [12] shows that the magnetic behavior of the given compound can be described as a short-range(localized) antiferromagnetic interaction, which confirms the assumption made.

According to Mossbauer data, the magnetic ions of the non-doped and Sr-doped Aurivillius phases with m < 5are distributed over the perovskite-like block more orderly. Magnetometric results show the greatest deviation fromparamagnetic behavior particularly for these samples. Taking into account the fact that the exchange interactionis most strongly manifested in the nearest magnetic ion neighbors, it can be concluded that in the case whenantiferromagnetic fluctuations exist in the system, they should be localized predominantly inside these blocks.


FIG. 7. Magnetization curves of (Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ measured at 2K (a) and300K (b). The curve number corresponds to the sample number (Table 1)

FIG. 8. Temperature dependences of inverse magnetization (a) and inverse magnetization (b) of(Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ . The curve number corresponds to the sample number (Ta-ble 1)

Thus, the magnetic characteristics of the structures based on the Sr-doped Aurivillius phases have been obtainedand found to be associated with the presence of a paramagnetic system formed by the magnetic moments of ironions, between which, there exists an exchange interaction of the antiferromagnetic type. The features of themagnetic behavior of these phases can be presumably explained by magnetic fluctuations.

4. Conclusions

The features of the solid-phase formation of the layered perovskite-like oxides of the Aurivillius phase type(Bi1−xSrx)m+1Fem−3Ti3O3(m+1)−δ (x = 0.0 – 0.7; m = 4 – 7) with a perovskite-like block ≈2 – 3 nm thickhave been described. The optimal conditions for the production of ceramic-based materials have been determined.The degree of isomorphous substitution in the bismuth sublattice was found to decrease along with the increaseof m and, accordingly, of h. The formation of a layered perovskite-like structure of the Aurivillius phases witha predetermined stoichiometry occurs with a gradual increase of m, while at m > 5 and h > 2 nm, isomorphoussubstitution leads to an increasing amount of impurities in the reaction system.

Thermal analysis data showed that the temperature for the initiation of materials decomposition gradually de-creases along with the increasing m, and the Curie point and the sintering onset temperature are in one temperaturedomain.


The results of the Mossbauer and magnetometric studies made it possible to make an assumption that themagnetic ions in the internal positions of a perovskite-like block are engaged in an antiferromagnetic-type exchangeinteraction that affects the magnetic properties of the studied materials. The effect of isomorphous substitutionin the bismuth sublattice on the nature of the iron ions distribution and the magnetic behavior of the Aurivilliusphases is less pronounced. The features of the magnetic behavior of the obtained materials can be presumablyexplained by magnetic fluctuations.

Acknowledgement

This work was financially supported by the Russian Foundation for Basic Research (Grant No. 16-03-01056).

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